The present invention relates to an extremely sensitive photoconductive member for electrophotography.
Materials for electrophotographic photoconduct members are Se, SeTe, ZnO, CdS and the like. A photoconductive member may be prepared by the vacuum evaporation of Se over an electrode. A ZnO photoconductive member is provided by applying fine crystals of ZnO dispersed in binder resins over an electrode. A CdS photoconductive member is provided by applying fine crystals of CdS dispersed in binder resins over an electrode and forming an insulating layer over the CdS layer.
In electrophotography, the photoconductive members prepared in the manner described above are used as follows. First the photoconductive member is uniformly charged, and a light image of an original to be reproduced is focused on the uniformly charged surface so that an electrostatic latent image may be formed. The latent image thus formed is developed with the toner, and the toner image is transferred on a suitable recording medium. Thus a copy is reproduced.
When an electrostatic latent image is formed, the photoelectric efficiency .eta. is given by .eta.=.DELTA..sigma./Fo where Fo=a number of photons impinged on a unit area; and
.DELTA..sigma.=the charge per unit area of the number of surface charge ions in response to the impingement of a light image.
The sensitivity of the photoconductive member is in general expressed in terms of the photoelectric efficiency defined above.
However the prior art Se, SeTe, ZnO and CdS photoconductive members have a photoelectric efficiency which is always less than unity. Furthermore it is not possible to increase their photoelectric efficiency to above 1.
U.S. Pat. No. 3,003,849 discloses a xerographic plate of high quantum efficiency of the type wherein over a base are formed an electrode, an electroluminescence layer, a transparent electrode layer and a photosensitive layer. When an ultra-violet light image is fucused on the base of this plate, the light is absorbed in the electroluminescence layer so that the carriers are excited. Under a high electric field produced by the surface potential, the excited carriers cause avalanche breakdown with the resultant increase in carriers. These carriers disappear after emitting light. Part of the light thus emitted is pumped into the photosensitive layer so that amplification of the incident light results. The exposure of this xerographic plate must be made with ultra-violet light; that is, no white light may be used in the exposure.
British Patent No. 833,188 discloses a light amplifier and storage device wherein a photoconductive layer and an electroluminescence layer are sandwiched between two transparent electrodes. However this device is used only for amplifying light by impressing AC voltage between the transparent electrodes. That is, the device is not used as a photosensitive member or plate.